Engineers typically design high-pressure oil field plunger pumps in two sections: the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. Commonly used fluid sections usually comprise a plunger pump fluid end housing with multiple fluid chambers, each chamber having a suction valve in a suction bore, a discharge valve in a discharge bore, an access bore, and a plunger in a plunger bore, plus high-pressure seals, retainers, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid end housing fluid chamber showing its connection to a power section by stay rods. A plurality of fluid chambers similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid section housing schematically illustrated in FIG. 2.
Valve terminology varies according to the industry, e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element or valve body. In the present application, however, the term “valve” includes other components in addition to the valve body, e.g., various valve guides to control the motion of the valve body, the valve seat, and/or one or more valve springs that tend to hold the valve closed, with the valve body reversibly sealed against the valve seat.
Each individual bore in a plunger pump fluid end housing is subject to fatigue due to alternating high and low pressures which occur with each stroke of the plunger cycle. Conventional fluid end housings, also referred to as Cross-Bore blocks, typically fail due to fatigue cracks in one of the areas defined by the intersecting suction, plunger, access and discharge bores as schematically illustrated in FIGS. 3A-B.
To reduce the likelihood of fatigue cracking in the high-pressure plunger pump fluid end housings described above, a Y-block housing design has been proposed. The Y-block design, which is schematically illustrated in FIG. 4, reduces stress concentrations in a plunger pump housing such as that shown in FIG. 3A by increasing the angles of bore intersections above 90°. In the illustrated example of FIG. 4, the bore intersection angles are approximately 120°. A more complete cross-sectional view of a Y-block plunger pump fluid section end housing is schematically illustrated in FIG. 5.
Both Cross-Bore blocks and Y-blocks have several major disadvantages when used to pump heavy slurry fluids as typically utilized in oilfield fracturing service. A first disadvantage is related to the feeding of the plunger bore cavity on the suction stroke of the pump. Upon passing through the suction valve, the fluid must make a 90 degree turn in a Cross-Bore housing, or a 60 degree turn in a Y-block housing, into the plunger bore as illustrated in FIG. 6. This change in the direction of the heavy fluid robs the fluid of kinetic energy, hereafter referred to as fluid energy.
Fluid energy is normally added to the fluid by small supercharging pumps upstream from the plunger pump. Fluid energy is necessary to overcome fluid inertia and ensure complete filling of the inner pump cavity or volume on the suction stroke. If the fluid could possibly enter the housing inner cavity or volume in a linear or straight path, less fluid energy would be lost.
The second disadvantage of Cross-Bore blocks and Y-blocks relates to the large intersecting curved areas where the various bores intersect. Because the suction bore above the suction valve is almost as large as the plunger bore, the intersection area of the suction bore with the plunger bore is particularly large as illustrated in FIGS. 3A and 3B. While the intersection of the suction bore and the plunger bore is notably large, the intersection of the discharge bore and the plunger bore is almost as large.
As shown in FIGS. 7A and 7B, the intersecting cylindrical sections result in intersection curves that focus or concentrate the stresses generated by the internal pump pressures into a very small area. This small area is located at the bore intersection near the plane formed by the axis of the plunger and suction or discharge bore cylinders at the finite point of the intersection of the two cylinders. Because the intersection curve changes slope through three-dimensional space, this intersection cannot be easily chamfered or filleted by conventional machining techniques that would mitigate these stresses to a smaller extent. Indeed, complex computer finite element stress analysis calculations indicate that chamfering or filleting the corner intersection has minimal effect on reducing the stresses at this corner intersection.
The amount of stress at the intersecting bores of conventional fluid end housings is defined by the magnitude of the “Bore Intersection Pitch” as illustrated in FIGS. 3A, 3B, and 4. Any geometry that reduces the “Bore Intersection Pitch” will reduce the stress concentrations in the fluid end and increase the life of the fluid end by mitigating cyclic fatigue failure. Y-Block fluid end housing designs, such as those illustrated in FIG. 4, do reduce this pitch, but the reduction is insufficient to prevent cyclic fatigue failure of the fluid end housing when subjected to high pressure and long pumping cycles.